Medical implants are used for reinforcing or entirely replacing damaged organs of the human body. An ideal material for this purpose should be neutral chemically, biocompatible with the tissue of a given organ, and resistant to corrosion. One of the materials most suitable for the fabrication of medical implants is chemically pure titanium, whose only drawback is its low mechanical strength since its ultimate tensile strength does not exceed 400 MPa and yield stress is 380 MPa. A much higher mechanical strength is shown by titanium alloys, such as e.g. the Ti—6V—4Al alloy, containing vanadium and aluminum, which has earlier been developed for the purposes of aircraft structures. The ultimate tensile strength of this alloy is 945 MPa and the yield stress is 817 MPa. The use of this alloy for the fabrication of medical implants was disclosed in U.S. Pat. No. 4,854,496, but, as it has later appeared, vanadium, an element harmful for the human body, migrates into the surrounding tissues.
A solution of the problem how to strengthen pure titanium without introducing to it harmful alloying elements was disclosed in U.S. Pat. No. 6,399,215 where the billet of coarse-grained titanium was subjected to many passes of hot equal channel angular extrusion (ECAE) followed by cold plastic deformation. These treatments gave ultra-pure fine-grained titanium with the average grain size between 250 to 300 nm, ultimate tensile strength ranging from 860 to 1100 MPa, and the yield stress from 795 to 1050 MPa.
The method of plastic deformation of metals, known as the hydrostatic extrusion, has been used since over 100 years (e.g. U.S. Pat. No. 524,504). In this method, the billet (material to be extruded) is placed in the working chamber filled with a pressure transmitting medium. At its one end, the chamber is closed with a piston and at its opposite end it is closed with a die whose shape is tailored to the required shape of the final product. When moving deep into the chamber, the piston compresses the pressure transmitting medium and thereby increases the hydrostatic pressure in the chamber. After the critical value of the pressure, characteristic of the given material, is reached, the billet material begins to be extruded through the die forming the final product. One of the important parameters of the hydrostatic extrusion process is what is known as the reduction ratio R which represents the degree of the reduction of the transverse cross-section of the billet and is defined as the ratio of the transverse cross-section surface area of the billet before the extrusion to the transverse cross section surface area of the final product after the extrusion.
Hydrostatic extrusion of titanium in the laboratory scale was reported in the publications by W. Pachla et al. entitled “Nano-structuring of metals by hydrostatic extrusion” [Proc. of 9th Int. Conf. on Metal Forming EMRS 2006 Eds. N. Juster, A. Rosochowski Publ. House Akapit 2006, pp. 535-538], and by W. Pachla et al. entitled “Nanocrystalline titanium produced by hydrostatic extrusion” [Journal of Materials Processing Technology, 2008 vol. 205, pp. 173-182]. The authors obtained a titanium wire with a diameter of 3 mm, an average grain size of 47 nm, ultimate tensile strength of 1320 MPa and yield stress of 1245 MPa. These parameters were however only achieved after as many as twenty consecutive extrusion passes and the quality of the wire surface was unsatisfactory for industrial applications. Other papers such as those published by K. Topolski et al. entitled “Hydrostatic Extrusion of Titanium—Process Parameters” [Advances in Materials Science vol. 7, no 4(4), 2007, pp. 114-120], H. Garbacz et al. entitled “The tribological properties of nano-titanium obtained by hydrostatic extrusion” [Wear 263, 2007, pp. 572-578], Topolski et al. entitled “The influences of the initial state on microstructure and mechanical properties of hydrostatically extruded titanium” [Solid State Phenomena Vol. 140, (2008), pp. 191-196], Topolski et al. entitled “Surface modification of titanium subjected to hydrostatic extrusion” [Inżynieria Materialowa Nr. 3, (2010), pp. 336-339, and H. Garbacz et al. entitled “Fatigue properties of nanocrystalline titanium” [Rev. Adv. Mater. Sci. 25 (2010) pp. 256-260] reported on experimental works which gave titanium wires with ultimate tensile strength between 1070 and 1140 MPa and yield stress between 890 and 1070 MPa, obtained after ten to twelve consecutive hydrostatic extrusion passes. None of the publications, cited above, suggests that it is possible to obtain titanium with similar or better properties when the number of the extrusion passes would be diminished at least by half. Two of the mentioned above publications (i.e. K. Topolski at al. “Hydrostatic Extrusion of Titanium—Process Parameters” and “Surface modification of titanium subjected to hydrostatic extrusion”) disclose also that prior to hydrostatic extrusion, titanium was covered with aluminum using the magnetron sputtering method, which permitted reducing significantly the maximum extrusion pressures and decreasing the wear of the die.